There Never Are Guarantees

The 4400's third season ends in a way that would serve for the end of the series itself. Smart, but fortunately there will be a fourth season next summer.
In this final episode, we finally see the confrontation : Isabelle vs everyone else. But let's take things from the start.
Jordan Collier films Devon taking a shot of promycin and urging the viewer to do the same in order to get a 4400 ability. The video is then circulated all over the internet. Tom and Diana, who are looking for the missing promycin, manage to track down Devon only to find her dying in Shawn's arms. The shot of promycin was fatal to her. Shawn, arrested, does not betray Collier's plan...until Ryland brings in Isabelle to question him personally. It is clear that Isabelle has replaced Shawn with power as her infatuation. She ends up leaving him hurt and unconscious but learns the location of the promycin. Accompanied by Ryland's troops, Isabelle arrives at the location soon and finds what she thinks is Jordan Collier along with the promycin. But it's a trap : the promycin is fake, and Jordan Collier is a disguised 4400 prepared to martyr himself for the cause. He gives Collier's message to Isabelle, "This is a new world, and there's no place for you in it", and activates an explosive device which kills Ryland's men, injures Ryland but only manages to anger Isabelle. Furious, she plans to attack the 4400 center, kill everyone in there to force Collier to appear. Ryland, seeing her madness, informs NTAC.
Tom knows it's time to use the shot given to him by the future. He and Diana arrive at the center just as Isabelle locks herself in Maia's class. Would she be able to murder children? Considering she has never been one, I don't find it hard to believe. She never gets the chance because Diana empties her weapon on her, and Tom hurries to use the syringe. Unfortunately, Isabelle recovers very fast and stops him. She incapacitates Diana with a move of her hand and in seconds the roles are reversed. Tom is about to be strangled by Isabelle and the syringe is out of his reach. It's at this time, that Richard appears and tries to convince Isabelle to stop her murderous urge. She knows it's too late for her to change and ignores him. Richard's telekinetic abilities, regretfully, come into play. Isabelle is shocked when the syringe flies into her neck. She convulses for a moment but comes around intent on retaliating and killing her own father. But the syringe's effect wasn't supposed to kill her. Her powers are gone forever. Tom can't risk it, however, and shoots her on the shoulder to make sure. Isabelle ends up in the hospital, next to Shawn.
The episode ends with several important events: Kyle is ready to inject himself with promycin. Alana is being kidnapped from the future. Diana chooses to quit NTAC and move to Spain with Ben and Maia, while her sister decides to give herself a dose of promycin. "There are no guarantees", the distributor tells her. "There never are" she thinks walking away. Then, the distributor turns to the viewer and offers promycin...would you take it?